Final answer:
Alkynes react with hydrogen halides (HX) in an electrophilic addition mechanism, first forming vinyl halides and potentially dihalides with excess reagent. Mild conditions are generally enough for halogen addition, but hydrogenation may require high pressure and a catalyst.
Step-by-step explanation:
Alkynes react with HX (hydrogen halides) through a mechanism known as electrophilic addition. This reaction involves the addition of HX across the alkyne's triple bond, which is composed of one sigma (σ) bond and two pi (π) bonds. When reacting with hydrogen halides, the alkyne first forms a vinyl halide, and if the reagent is in excess, a secondary addition can occur, leading to a dihalide derivative. For example, when acetylene (—C≡CH) reacts with hydrogen bromide (HBr), the hydrogen atom of HBr will add to one carbon of the triple bond, and the bromine will add to the other, forming vinyl bromide as an intermediate product. If excess HBr is present, the reaction will continue to add another equivalent of HBr to form 1,2-dibromoethane.
The conditions for this reaction are typically mild, and no catalyst is required for the addition of halogens to alkynes. However, for the addition of hydrogen (hydrogenation), high pressure and a metal catalyst such as platinum or palladium may be necessary to achieve full conversion to an alkane. Hydrogenation of alkynes can proceed stepwise, first yielding an alkene before full saturation to an alkane takes place.